This invention relates to an improved fuse for electrical circuit protection. It has particular application to an easily manufactured, high precision, high performance sub-miniature fuse of the type which may be used to protect printed circuit boards and components.
The term "sub-miniature fuse" as used herein means a fuse, including its fusible element and its container, having a width of less than one-tenth inch, to allow multiple fuses to be mounted on tenth-inch centers on a printed circuit board. Ideally, the fuse has a volume of less than 0.01 cubic inches. It will be understood that the sub-miniature fuse may be mounted in additional external packaging and may include leads extending beyond the dimensions of the fuse body itself.
In the past, sub-miniature fuses have been made by suspending a small fusible wire between the ends of glass or ceramic tubes. Electrical contact is made to the fusible wire by metal end caps which are soldered or mechanically crimped to the fusible element. The whole assembly is held together by crimping the end caps to the glass or ceramic tube.
When axial leads must be affixed to the end caps, for mounting the fuse on a printed circuit board, the fuse body and end caps must be held together with a plastic material to give the assembly enough strength to be handled normally.
The traditional sub-miniature fuse assembly as described has many shortcomings.
The physical dimensions of a fuse to be mounted on a printed circuit board must be as small as possible. When the length of the fusible wire is made short, its diameter must be decreased to maintain the required fuse characteristics. In some cases, the fusible wire must be as small as 0.0003 inches in diameter. Such small wires are extremely hard to assemble into a traditional sub-miniature fuse and cause the cost of manufacturing to be high. As a result, very low current fuses are not practical because of the small size wire required. Moreover, existing sub-miniature fuses are specifically designed for a particular mounting, and are not easily modified for mounting by axial wire leads, surface mounting, or semi-conductor type inline mounting.
The typical sub-miniature fuse using a wire fusible element cannot be controlled to extremely close circuit interrupt characteristics because of variations in fusible wire diameter, composition and free length. Crimping and solder type electrical connections to the fusible wire element are notoriously inaccurate methods for controlling the free wire length.
Furthermore, the traditional construction is not hermetically sealed. Although some other constructions provide a plastic seal, most do not provide the truly hermetic seal which can be provided only by a proper glass-to-metal seal. Therefore, they can neither contain a given gas composition nor protect the interior from external gas and vapor contamination. As a result, the electrical characteristics of the traditional sub-miniature fuse are subject to change with age and environmental conditions.
With the traditional sub-miniature fuse construction, high current and high voltage fuses are not practical. The short length of fusible wire and close proximity of metal end caps causes a very energetic conductive plasma to establish itself inside the fuse body during high voltage and high current fault interruption. The resulting vaporized metal plasma arc heats the interior of the fuse rapidly and generates high internal pressures which cause the device to explode destructively, thereby putting in jeopardy other components on the printed circuit board. Both physical damage and fire hazards can result from such an explosion.
The traditional construction is inherently weak when subjected to axial pull loads because only the encasing plastic holds the end caps and axial leads in place. The external plastic cannot be made heavy enough to support typical loads without increasing the external fuse dimensions beyond reason.
The need to hold traditional sub-miniature fuses together with external plastic coatings makes visible inspection of the interior, to determine whether a fuse has blown, virtually impossible.